Hey, Mike. To answer your questions:
1. I do not believe that the 663 will have any trouble with 6-Ohm speakers. You shouldn't have to set the receiver up to handle that size. You are using them for a second zone and probably don't have a sub in that zone, so I'd set them as "large."
2. Standard banana plugs should be just fine, as long as you can connect the small wires to the plugs that you have. If your wall plates take bare wire, that's another good choice.